This invention relates to assembly of turbine bucket assemblies having tangential entry dovetails onto a turbomachine bucket wheel and, more particularly, to a method for assembly such that relative motion between adjacent tangential entry dovetails and/or the bucket wheel is minimized at operational speed and temperature.
In an axial fluid flow turbine, such as a steam turbine, the blade elements, or vanes, may be secured to a dovetail assembly to form a bucket assembly. These bucket assemblies are mounted on the rim of a turbine wheel such that the bucket assemblies are radially inwardly inserted one at a time at a predetermined location on the rim, and are then circumferentially positioned in dovetail mounting grooves in the rim until there is a full circumferential row of bucket assemblies on the rim. With such a construction, the dovetail based portions of the bucket assemblies often have lateral planar faces lying in a plane parallel to a radial plane which abut similar faces of adjacent bucket assemblies, so that each bucket assembly is held circumferentially in place by bucket assemblies pressing against it on either side thereof. With this construction, it is desirable to have a tight structure in order to assure the correctness of the overall assembly, to determine the natural vibration frequencies, and to prevent any looseness which may lead to fretting or wear, resulting in undesirable consequences such as reduced fatigue strength of the material constituting the bucket assemblies or mating wheel.
In certain turbomachine applications, the aforedescribed type of turbine wheel construction may be subject to a phenomenon known as "arch binding", which causes a gradual increase in the diameter in the wheel to which the buckets are attached, resulting in increased compressive forces between dovetail assemblies. Apparatus for reducing the affects of arch binding by reducing the tangential compressive forces present in a bucket wheel is described in U.S. Pat. No. 3,084,343--Rubio et al, which is assigned to the present assignee. However, it is believed that the detrimental affects due to arch binding are not manifested until the bucket wheel experiences operating temperatures above about 700.degree. F. Arch binding is also a function of the materials constituting the bucket assembly and wheel and their respective coefficients of thermal expansion. Arch binding is more likely to occur if the coefficient of thermal expansion for the bucket assembly is greater than the coefficient of thermal expansion for the wheel.
For certain bucket wheel applications, such as for operating at elevated temperatures and wherein the wheel coefficient of thermal expansion is greater than the bucket coefficient of thermal expansion, it may be desirable to increase the circumferential, or tangential compressive, force between the dovetail based portions of the bucket assemblies. One such apparatus for increasing the circumferential force exerted on a bucket wheel is described, especially with respect to FIG. 5 thereof, in U.S. Pat. No. 3,721,506--Anderson, which is assigned to the present assignee. Although the apparatus of the Anderson patent may be used in appropriate cases, it is desirable to increase the circumferential or tangential force between base portions of bucket assemblies circumferentially disposed on a dovetail of the rim of a bucket wheel without using additional hardware.
Accordingly, it is an object of the present invention to provide a method for assembling a plurality of bucket assemblies onto a wheel of an axial fluid flow turbine such that residual circumferential tightness is maintained between adjacent bucket assemblies at operating temperature and speed.